In a radio communication system including a portable telephone system and a mobile telephone system, and other communication systems, an antenna switch is used. The antenna switch has an antenna switch portion which selects one of two antennas, and a transmit/receive switch portion which switches the selected antenna between transmission and reception.
The antenna switch portion switches between a small inner antenna mounted in a communication apparatus, and an external antenna.
The transmit/receive switch portion switches the selected antenna either to an output of a transmitter or to an input of a receiver. This is in high speed operation when a digital radio communication system is used.
FIG. 10 shows a prior antenna switch having a conventional antenna switch portion. In FIG. 10, the symbols C40-C43, C50-C51, C53, and C60-C62 are a capacitor, L40, L50 and L60 are a coil, DA1, DA2, DB1 and DB2 are a PIN diode, SL41 and SL42 are a strip line, Vb1 and Vb2 are a terminal for applying bias current for switching ON and/or OFF said PIN diode, Tx is a transmit terminal, Rx is a receive terminal, ANT1 is a terminal coupled with a first antenna, and ANT2 is a terminal coupled with a second antenna.
First, the basic operation of an antenna switch circuit having an antenna switch portion is described with the example of a transmit/receive switch portion. In the antenna switch in FIG. 10, when bias current is applied to a bias terminal Vb1, PIN diodes DA1 and DA2 are switched ON so that each of those PIN diodes are conducted with small resistance. Then, a strip line SL41 is grounded at the end coupled with the PIN diode DA2. When the length of the strip line SL41 is 1/4 wavelength of an input signal frequency at the transmit terminal Tx, the impedance at the other end of the strip line SL41 coupled with the PIN diode DA1 is infinite, due to 1/4 wavelength resonance of the strip line SL41. Thus, the portion of an input signal applied to the transmit terminal Tx passes the PIN diode DA1 to a capacitor C42. The leakage of a signal from a transmit terminal Tx to a receive terminal Rx is called an isolation.
Next, when the bias current at the bias terminal Vb1 is turned OFF, the PIN diodes DA1 and DA2 are switched OFF, and the resistance of those PIN diodes becomes infinite. Then, the transmit terminal Tx is isolated from the capacitor C42 by the PIN diode DA1. On the other hand, the signal at the capacitor C42 comes to the receive a terminal Rx through the strip line SL41, since the PIN diode DA2 is in OFF state.
The isolation is important in a transmit phase between a transmit terminal Tx and a receive terminal Rx, but it is not important in a receive phase, since transmit power is much higher than receive power. The operation of an antenna switch portion is not described since it is similar to the above described operation of the transmit/receive switch portion.